The perforated granulation plates used in plastic extruders are subject to intense wear on the work surface on which granulation knives pass and it is customary to cover this surface with wear-resistant material, at least in the region of holes which form the plastic composition into strands. The attachment of the wear-resistant material, which, as a rule, is in the form of a shaped member having one or more holes, is generally effected by soldering, which makes it necessary for the materials of the parts to be connected to consist of metal or contain metallic components. The latter is true, for instance, in the case of sintered tungsten carbide. Due to the high demands which are made on such solder connections, generally only noble metals are used as the solder. Furthermore, the soldering must be effected in a specifically defined atmosphere which as in an inert-gas or vacuum furnace. The removal of the shaped members in the event of repairs requires considerable work and expense as a result of the diffusion bond produced by the soldering.
A perforated granulation plate of the type described above is known from U.S. Pat. No. 3,271,822. For protection against wear, the work surface of the perforated granulation plate is provided with square shaped pieces of sintered tungsten carbide. The shaped members have a hole which is coincident with a bore in the perforated granulation plate for the passage of the plastic mass and the shaped members are inserted into a circular recess in the surface of the perforated plate. The bottom of the recess is covered with thin strips of silver solder and copper. The spaces present between the vertical walls of the shaped members and the walls of the recess are also filled with strips of the solder material. The soldering is effected by heating the entire perforated granulation plate in an inert-gas furnace to the melting point of the solder material. The production of the means for providing wear protection is therefore very involved and expensive in the case of the known perforated granulation plate.
It is known from U.S. Pat. No. 3,804,392 for double-shaft worm extruders having a bore which is of figure-eight shape in cross section, to provide the inner surfaces thereof in part with wear-resistant inserts. These inserts have a trapezoidal cross section and are arranged in dovetail-shaped longitudinal grooves. Since the cross sections of the inserts and of the grooves coincide, the inserts can be pushed into the grooves only from one end.